Simple, Analog V & A Meter Install on Small System Questions

Re: Simple, Analog V & A Meter Install on Small System Questions

The terms you will want to understand is:

Parallel: For measuring voltage you will want to place the meter in parallel with the load, source or whatever you will want to measure the voltage of...

Series: You will want to place the meter in between the load and the source (break a wire and insert the meter) to measure current.

Shunt: Used to measure current higher than a normal meter would be capable. Insert shunt in series (as above). Then use a Volt Meter (typically one that can measure millivolts, or 0.001 or 0.0001 volts minimum resolution and up to 1 volt or so). The shunt converts the current into a voltage reading.

Now the warnings:

Connecting any meter to high current (or high voltage) circuits require you to be very careful about shorting wires. Shorts can very easily start fires and completely destroy your RV/HOME/Etc. If you are installing the meters permanently, understand where to use fuses and protect the wires from shorts.

A voltage meter can be damaged by attempting to read voltages that are too high (fire, etc.).

A current meter can be damaged by attempting to read currents that are too high (or even very high voltages too--fire, etc.). A shunt is needed anytime the current is greater than the rating of the meter (or if you have a voltage meter and wish to measure current).

Voltage meters draw power and leaving an inexpensive analog meter connected all the time can actually discharge your battery over time (from a few percent to 10% of your battery capacity per month--depending on the meter and the size of your battery bank). For a panel meter you will probably want a push to test or toggle switch to only measure voltage when you are there.

A current meter (unless it has a back light or something odd) is not going to consume any power over time--you can leave it connected all the time.

Panel meters are sometimes intended to be used with shunts or other external scaling device (such as a pair of resistors) so the markings on the scale are not always indicative of the actual function of the meter (for an example a panel meter that reads 0-100 amps that has small gauge connections/wires is probably a volt meter intended for use with a current shunt).

A couple of basic electrical formulas that should be helpful:

V=I*R (voltage = current * resistance)
I=V/R
R=V/I

P=IV (power in watts = current * voltage)
P=I^2*R (current "squared" times resistance)
P=V^2/R

Watt*Hours (measure of the amount of work--or power over time--e.g. 100 watt light bulb for 10 hours would be 1,000 watt hours or 1 kilowatt hour worth of work done or required).

AMP*Hours (manytimes used to measure the capacity of a battery based system. Similar to WHr but does not include voltage (so you have to know if the system is a 12 volt, 24 volt or whatever size system).

A 100 AmpHr battery would be able to supply a 1 amp load for 100 hours (12 watt load for 100 hours) or a 50 amp load for 2 hours, etc. (note that I am being really loose with my definitions here--a battery can deliver more total energy if its loads are small and over time. The observed capacity of a 100 amp*hr battery supplying 200 amps will be much less than 30 minutes.

Note that a standard car battery should probably not be cycled for more than 20% of capacity (100 amp*hr battery should only run 1 amp for 20 hours) or risk the battery failing in months, or even a few weeks.

A true storage battery (hopefully like your RV system) should not be cycled for more than 50% of capacity (100 amp*hr would be used as a 50 amp*hr battery).

I hope the above is useful.

-Bill

Re: Simple, Analog V & A Meter Install on Small System Questions

The 15v Voltage meter is the correct range. It should be install + terminal of meter to + wire of your RV. The meter's - terminal should be connected to the metal frame of your RV (or ground wire if one is near by). The switch would typically be installed by breaking the + lead of the meter lead.

Voltage meters--for what you are doing--would never use a shunt. There is something called a resistor divider (that is not a shunt) that can be used for high voltages or in special situations--but would almost never be used by an RV'er...

The current meter should be designed to work without a shunt (really need to read the specs of the meter--or sometimes the information in printed in very small type on the front of the meter). At those small currents of a 15 to 60 watt panels, it would not hurt much to just connect the meter and see if it works correctly or not.

And you will not need a switch or shunt for your current meter--it is connected all the time.

If you have a simple solar charge controller, you can place the meter either between the charge controller and the battery, or between the panels and the charge controller. Assuming that your RV is negative ground--I would tend to place the current meter in the positive lead from the panel or to the battery... Much less confusion than the negative leads, especially when working with a charge controller (where the charge controller's negative lead does not carry the load).

The current meter can go in either the positive or negative lead of the solar panel. It does not matter. Simply cut one wire from the solar panel and insert the meter leads. If the meter reads backwards, just swap the leads and try again. If you have more than one panel, connect all of them together (+ to + and - to -), then put the meter in the one lead to the battery and you will measure the total current supplied.

In general, also use a fuse (or connected to an existing fused circuit in your RV) to protect your wiring from fire. It is not difficult to do correctly and the problems if the wiring is not fused correctly are huge if there is a short later. Lead Acid storage batteries can output HUGE AMOUNTS OF CURRENT--much more than you are used to in your AC Home wiring (think large arc welder type currents). Home AC wiring was installed and inspected by professionals with fire safety in mind--An RV power system is very easy to bypass fuses and create safety issues. You would probably never think of opening your main breaker box and connection wires directly to the 240 VAC coming directly off of your power pole--but many folks will do almost exactly the same thing when connecting wires to a large 12 VDC battery.

My normal disclaimer--It is very hard to understand the knowledge of a poster on an Internet Board when answering questions... I am not trying to be difficult, but you may wish to get a friend or pay somebody to help with your first installation so that you understand the basic safety requirements and can ask questions.

Yes, the connections are simple--but the downside of making a mistake can be property and life threatening when dealing with these power levels.

-Bill

Re: Simple, Analog V & A Meter Install on Small System Questions

AZ,

You sound good to go... If you want to think of the meters in a different way... Think of connecting a light bulb instead.

Connecting a bulb from + to - will glow whenever there is voltage present.

Connecting a bulb by breaking a wire--it will glow only when current is flowing in the wire.

That is pretty much all there is to it.

Good Luck and have Fun! (boy, I miss camping--my wife is a city girl).
-Bill